At the present time, it is common to permit oil and gas wells to flow under their own natural pressure as long as they will do so and then to apply a mechanical reciprocating pump to complete the removal of the liquids. This method, although in general use, is cumbersome and unsatisfactory. Because suction will only raise oil for a distance of some thirty-five feet, it is necessary to have the pump near the bottom of the well so that it can exert pressure instead of suction on the liquids coming out of the well. This involves the use of pump rods of lengths of 5,000 feet or greater. In many instances when the pump plunger or the valves become worn, it is necessary to remove the pump from that depth to replace the worn parts. Furthermore, the collars on the pump rod wear rapidly and all the pump parts do likewise because of the small particles of grit that remain in the liquid and the whole device is mechanically inefficient because of the relatively long pump rods that must be reciprocated to perform the pumping operation.
When the natural flow of liquid from a well has ceased or becomes too slow for economical production, artificial production methods are employed. In many cases, it is advantageous, at least during the first part of the artificial production period, to employ gas lift. Numerous types of equipment for producing liquid by gas lift are available, but they all rely upon the same general principles of operation. In the usual case, dry gas consisting essentially of methane and ethane is forced down the annulus between the tubing and the casing and into the liquid in the tubing. As the liquid in the tubing becomes mixed with gas, the density of the liquid decreases, and eventually the weight of the column of the gasified liquid in the tubing becomes less than the pressure exerted on the body of liquid in the well, and the flow of liquid occurs at the surface. While, in some cases, the dry gas may be introduced through the tubing so as to cause production through the annulus, this is not preferred unless special conditions are present.
One known gas lift technique injects gas into the casing, which has been sealed or packed off at the bottom of the hole relative to the production tubing. A gas lift valve is placed in the production tubing at the production level, and the gas lift valve permits the gas to be injected into or bubbled very slowly into the liquid being produced from the well. This gas then makes the liquid in the production tube somewhat lighter and, hence, the natural formation pressure will be sufficient to push the liquid up and out of the well. This means that the well can be produced at a greater rate. This gas lift technique is known as continuous gas lift.
A further adaptation of this gas lift technique is known as intermittent gas lift. In this technique, rather than letting the gas enter the production tube slowly, the gas is injected into the production tubing very quickly, in short bursts, thereby forming a large slug of liquid in the production tubing above the injected gas bubble. The gas bubble then drives the slug of liquid in the production tubing upwardly. The technique is repeated successively, thereby producing successive slugs of liquid at the wellhead.
Another type of gas lift tool involves a procedure where a string of production tubing extending from the surface to the zone of interest is provided with a number of gas lift valves positioned at spaced intervals along the length of the tubing. Gas is injected from the annulus between the tubing and the well pipe through the gas lift valves and into the tubing for the purpose of forcing liquid upwardly to the surface and ultimately into a flowline that is connected with the production tubing. Gas lift systems for liquid production are quite expensive due to the cumulative expense of the number of gas lift valves that are ordinarily necessary for each well. Moreover, each of the gas lift valves must be preset for operation at differing pressures because of the vertical spacing thereof within the tubing string and because the valves must function in an interrelated manner to achieve lifting of liquid within the tubing string.
In the past, various patents have issued relating to such gas lift systems. For example, U.S. Pat. No. 5,671,813, issued on Sep. 30, 1997 to P. C. Lima describes a method and apparatus for the intermittent production of oil. In this method, two production strings extend downwardly from a wellhead of an oil well to a point adjacent a producing region. The lower ends of the two production strings are connected by a coupling which allows a mechanical interface launched adjacent the wellhead of one of the production strings to descend along the production string through the coupling and upwardly through the other production string to displace oil from the production strings to a surge tank. High pressure gas is utilized to move the mechanical interface through the production strings and suitable valves are provided for controlling the flow of gas and oil through the production strings.
U.S. Pat. No. 5,562,161, issued on Oct. 8, 1996 to Hisaw et al. describes a method of accelerating production from a well. This method includes the steps of installing a venturi device within the well. A gas is injected within the annulus and introduced into the well. The venturi device creates a zone of low pressure within the well as well as accelerating the velocity of the production fluid so that the inflow from the reservoir is increased.
U.S. Pat. No. 5,407,010, issued on Apr. 18, 1995 to M. D. Herschberger teaches an artificial lift system and method for lifting fluids from an underground formation. This artificial lift system includes a production tubing through which the fluid is carried from the formation to the surface and a pressure reducer, such as a venturi, connected to the production tubing to artificially raise the level of the fluid in the production tubing above the static level associated with the head pressure of the fluid in the formation.
U.S. Pat. No. 5,217,067, issued on Jun. 8, 1993 to Landry et al. describes an apparatus for increasing flow in an oil well which includes an injection valve so as to enable gas to be injected and to cause the oil or other liquid within the well to be lifted to the surface. The valve has a valve body having an inlet at one end and an outlet at the other end which are adapted to be fitted into conventional production oil tubing. A gas injection port opens into the outlet of the valve body and there is at least one gas inlet opening in a side of the valve body. This gas inlet opening is connected to the gas injection port. This enables compressed gas to be sent down the well between the casing and the tubing and injected through the gas injection port and into the flow of oil.
U.S. Pat. No. 5,211,242, issued on May 18, 1993 to Coleman et al. describes a chamber in a well which is connected to two externally separate tubing strings to unload liquid which is applying backpressure against a formation so that the production of fluid from the formation is obstructed. Volumes of the liquid are intermittently collected in the chamber and lifted out of the well through one of the tubing strings in response to high pressure gas injected solely into the chamber through the other tubing string.
U.S. Pat. No. 4,708,595, issued on Nov. 24, 1987 to Maloney et al. describes an intermittent gas-lift apparatus and process of lifting liquids. This apparatus includes a chamber on the downhole end of a production tubing in communication with a sidestring tube. The sidestring tube is in communication with the high pressure gas stored within the casing and above and below a packer. A valve in the sidestring tube permits the entrance of a lifting gas into the chamber to lift the liquids flowing therein to the surface. A surface bleed-down system minimizes the pressure in the production tubing. This increases the pressure differential between the formation and the interior of the casing and lifting chamber during the operation of the apparatus.
German Patent No. 23 64 737, published on Jul. 10, 1975, teaches a compressed air lift pump for deep wells in which the pump has a number of stages one above the other. Liquid is raised by air from the reservoir of one stage to the reservoir of the next. Each stage has two air supply pipes which contain three-way valves operated by an electronic timer to admit and release air alternately.
Soviet Patent No. 1204-700-A teaches an intermittent gas lift system for a pump well which includes a tubing, a packer, a substitution chamber and intake valve, lift starter valves and working valves with a seal and a seat over a space connected to the chamber. The rising level of fluid in the chamber raises the float so as to close off ports and thus raise pressure above the diaphragm so as to clear the valve and transfer gas to the chamber. This gas forces the fluid into the tubing and uses a pressure gradient to hold the ports closed. Gas eventually enters the tubing after all fluid has been expelled, thus opening the two ports by lowering the float back down. Gas is removed entirely from the chamber by the incoming fluid.
Soviet Patent No. 570697 teaches an oil production facility including a displacement chamber, two strings of compressor pipes of which one is coupled to the surface drive. The gas from the chamber is recuperated and expanded. When one vessel is empty, fluid is drawn into the displacement chamber. The second vessel pumps oil over into the empty vessel so as to raise its pressure to the point required to drive the hole fluid over into the lifting string to the surface. Once the fluid in the chamber reaches the bottom of the lift string, the motor reverses so as to turn an electric shaft and compress the gas in the first vessel to repeat the process in a second hole.
A major problem with the aforedescribed artificial lift systems is that they do not work effectively in deep well and sour gas environments. In particular, at depths of greater than 10,000 feet, the temperature range encountered can be approximately 300 degrees Fahrenheit. As such, any mechanical pumping apparatus will not work effectively at such temperatures. At such great depths, the rod pump devices and submersible pump apparatus do not effectively deliver oil and gas to the surface. For example, at such great depths, the pump rod will have an extreme length which cannot be easily reciprocated back and forth. Furthermore, the cost associated with such a lengthy pump rod would not allow for efficient production. The high temperature and pressures encountered at such depth cause submersible pumps and hydraulic pumps to fail quickly.
In those systems in which the intermittent production of "slugs" of oil is utilized, such systems are ineffective at such depths. In each case in which a "slug" of oil is produced, the gas must be relied upon so as to deliver such a slug to the surface. At great depths, this can take a great deal of time so as to produce an economical amount of oil. Furthermore, the pressure and energy required so as to push such a slug to the surface may exceed the value of the actual production.
Production at such a depth is further complicated by situations in which a corrosive sour gas is encountered. This is particularly true in those cases in which oil and gas must be removed from Smackover wells.
It is an object of the present invention to provide an artificial lift system which works effectively at depths of greater than 10,000 feet.
It is a further object of the present invention to provide an artificial lift system which can operate in a high temperature environment at the bottom of the well.
It is another object of the present invention to provide an artificial lift system in which production from the liquid string occurs continuously without the need for transporting a "slug" of oil to the surface.
It is another object of the present invention to provide an artificial lift system which works effectively in highly corrosive sour gas environments.
It is another object of the present invention to provide an artificial lift system which can lift liquid volumes of approximately 500 barrels per day.
It is a further object of the present invention to provide an artificial lift system which can operate in a very "gassy"/high API oil gravity environment.
It is still a further object of the present invention to provide an artificial lift system which can handle saturated brines of greater than 200,000 parts per million.
The foregoing objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.